1. Field of the Invention
The present invention relates to a Tessor type lens system for use in a facsimile apparatus having a half angle of image of approximately 18.degree. and an aperture ratio greater than 90% with an f number of 4.5.
2. Description of the Prior Art
The Tessor type lens system has been generally used in the field of photography. However there are several problems in its application to the facsimile system.
One problem is that the photographic lens system which is adapted to infinity in the object distance can not be practically used for the facsimile systems which do not require the lens system to be adapted to infinity in the object distance.
Another problem relates to an astigmatism, since, in the facsimile system, a parallel plane such as translucent contact glass having a certain thickness is disposed between the lens and object, the phenomenum of astigmatism occurs.
Moreover the facsimile lens system is required to have a large aperture ratio as the facsimile system is designed to form an image of relatively narrow range at one particular time in contrast to a wide range in photography.
As a consequence of these problems inherent in the Tessor type lens system, it is extremely difficult to provide high contrast and a large aperture ratio while at the same time correcting various aberrations that exist.